The pathology caused by the ciliate Anophryoides haemophila was studied in experimentally infected lobsters. The first lesions due to the parasite were observed in gills and connective tissue from 9th week post-infection for the lower inocula and at week 4 for the 500,000 ciliate inoculum.. These lesions developed at all inocula (2000, 10,000 and 500,000 ciliates per lobster) and they were also present in both euthanized and moribund lobsters. The higher inocula resulted in higher numbers of parasites observed in sections of gills and in the connective tissue of other organs, with no penetration to the parenchyma of any organ. Furthermore, the highest inoculum of ciliates (500,000 per lobster) resulted in a shorter course of disease development and consequently, experimental lobsters died between weeks 4 and 6 post-infection. This was not observed at inocula of 2000 and 10,000 ciliates per lobster where mortality occurred 11-14 weeks post-infection.